The evaluation of soil and water conservation benefit of comprehensive management of small watershed is not only of guiding significance for promoting the comprehensive management of small watersheds,but also has important reference value for the assessment of green GDP in promoting the construction of ecological civilization in our country.In order to improve the efficiency of evaluating the benefits of soil and water conservation in small watersheds and to standardize the evaluation process.In this study,the basic data are small watershed topographic maps,land use distribution map,soil texture map and field survey data.The author uses relational database technology,spatial database technology and ArcGIS Engine secondary development technology,and together with the distributed hydrological model and revised universal soil loss equation, has developed a benefit evaluation system of soil and water conservation in small watersheds.The system can realize the six functions of data management,hydrological benefits evaluation,soil maintaining function evaluation,ecological service function evaluation,main channel evaluation and economic benefit evaluation.This study will elaborate on two main functions of soil maintaining function evaluation and main channel evaluation.The system is used for evaluating Puwa small watershed,which is the earliest one in carrying out the construction of ecological clean watershed in Fangshan District,Beijing.The results showed that the soil erosion intensity decreased significantly before and after the comprehensive management of small watershed.Land use change and vegetation restoration were the main influencing factors of soil erosion protection.The main channel hydro-geomorphology is generally good,poorly assessed channels are mainly distributed in the middle and lower reaches where human activities are frequent,and human activities are the main factor affecting the channel environment.